The first portion of the background is related to large modular battery systems that can be used to absorb regenerative braking energy. Some example systems are large battery systems used in battery powered electric locomotives, battery packs that will adsorb the motor braking energy of a crane.
In these cases, the cranes and locomotives have a known capital value and long life up to 30 years. What is challenging to a customer is determining the value and life of a chemically active battery or ultra capacitor cell (terms used interchangeably throughout this section).
In the case of battery powered locomotives, the cost of the battery could make up ½ the cost of the piece of equipment. This is a large capital expense and battery cells have a finite life much shorter than the 15 to 30 year life of many of these pieces of equipment. As the battery cells mature as a technology the life expectancy and performance of these cells is improving but the total cycle life of these cells is unknown so judging their value to justify these large capital purchases is proving to be a challenge.
How a battery cell is used has a significant impact on life expectancy. The higher the C-rate of the power flow in or out of a battery, the more cumulative damage each charge or discharge cycle subjects the battery cell to. C-rate is a term used to describe the ratio of instantaneous current in amps to the batteries energy capacity in amp hours. A typical 18650 battery cell used in a laptop computer could have an amp-hour rating of 3.4 amps. If this battery is charged at 1.7 amps, the it is being charged at a C-rate of 0.5.
It is the discharge and charge cycle caused cumulative damage cycles over time that continuously reduce a battery cells capacity to store energy. When a battery cell drops to a certain portion of its original energy capacity it is usually considered to be at the end of its useful life. In weight and range sensitive applications like light duty automobiles, this end of life energy capacity is 80%. For less weight-sensitive applications such as rail, a lower number like 60% could be acceptable.
Between charging and discharging currents, it is typically the charging currents that cause the most cumulative degradation per cycle. The relationship of C-rate to degradation is not linear. Degradation at a discharge current of 1C is greater than twice the degradation at a 0.5 C charge rate. When charging a battery overnight at C-rates of less than 0.5 the degradation is minimal. When charging the battery during a regenerative event in a locomotive, the C-rate may be as high a 2.0. It is the hard to predict life expectancy of the batteries that make their value as a capital asset so hard to determine. This is one of the major challenges to the introduction of batteries into rail equipment. What would be desirable would be an effective way to pay for batteries as a consumable like fuel rather than a capital equipment cost due to their shorter and variable life.
The second part of the background is related to the use of wireless power transfer systems (WPT) for rail applications.
The onboard electrification article published in July/August 2014 Steel Wheels (www.railpac.org) discusses a system for electrifying start and stop passenger rail by first adding a Zero Emissions Boost Locomotive (ZEBL) and then adding a wireless power transfer (WPT) system to it. One significant challenge with adding WPT is that one single 20-foot long WPT system would have to fit in between the rails and transfer power at 1.8 MW to transfer 20 kW-hrs in the average 40 second commuter rail stop.
Another significant challenge to WPT for rail applications is that conventional WPT systems require accurate alignment between WPT transmitter and receiver antennas.
The highest capacity individual WPT units currently available for transit buses and light rail applications are only capable of 50 kW and will be over 42″ in diameter, possibly 52″.
In Korea, there is a WPT system for moving trains that uses long sections of single loop coils much longer than the train it is charging. This system is very economical and can transfer high power levels, but is not practical for applications where people will be near or walking over the track such as in a switching yard or port area. Even some passenger stations have walk ways for passengers to walk across the tracks.
Locomotives operate with an air brake system that has been standardized over decades and while proven to be very reliable and safe, it is not practical to stop a train within a distance of plus or minus a few inches as conventional WPT system require.
What is desired is a practical way to spread out the receivers to absorb the total transmitted power at a lower intensity and without the need for precise stopping locations.